Aqueous coating compositions are used in a variety of applications in the automotive coatings industry. They advantageously provide reduced organic emissions, lower toxicity, and a reduced fire hazard. The aqueous coatings are, in general, “dispersions” or two-phase systems of a finely divided solid or liquid in a continuous medium. As used herein, “dispersion” refers to two-phase systems of one or more finely divided solids, liquids or combinations thereof in a continuous liquid medium such as water or a mixture of water and organic cosolvent. “Emulsion” as used herein refers to a dispersion of liquid droplets in a liquid medium, preferably water or a mixture of water and various cosolvents. Accordingly, the term “dispersion” will be used in this description to include an “emulsion.”
Aqueous coating dispersions may be used as electrodeposition coatings, primers, sealers, basecoats, enamels, and/or topcoats. Various resins and binders may be used in aqueous coating dispersions, including but not limited to, epoxy based resins, acrylic resins, polyester resins, alkyds, polyurethanes, polyurethane adducts, and the like. A “resin,” as used herein, includes a “material,” which is not formed of repeating units; an “oligomer” formed of repeating units and having a molecular weight <1000 g/mol; and, a “polymer” having repeating units and having a molecular weight >1000 g/mol.
Electrodeposition coating compositions and methods are widely used in industry today. In the electrodeposition coating process, electrically charged coating particles are plated or deposited out of an aqueous dispersion onto a conductive substrate. Electrodeposition can be anodic or cathodic; typically the article to be coated serves as the cathode. Electrodeposition or “electrocoat” processes are advantageous both economically and environmentally, due to the high transfer efficiency of solid coating to substrate and low levels of organic solvent. One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all portions of the metallic substrate provides maximum anticorrosion effectiveness.
Electrocoat baths usually comprise an aqueous dispersion of a film-forming resin, such as an epoxy or acrylic resin, having ionic stabilization. For automotive or industrial applications, in which hard electrocoat films are desired, the electrocoat compositions are formulated to be curable compositions. This is usually accomplished by including in the electrocoat bath a curing agent, also known as a crosslinking agent, that can react with functional groups on the resin under appropriate conditions (such as with the application of heat and/or UV radiation) and thus cure the coating. During electrodeposition, coating material containing an ionically-charged resin is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin and then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating dispersion migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure the coating.
Problems relating to aqueous coatings involve handling and preparation of the aqueous coating components and prepared dispersions. An aqueous coating dispersion that contains a significant amount of aqueous solvent—the aqueous coating can sometimes include from 50% to 80% water—adds considerable weight, volume, and cost in transportation. While a coating composition can be prepared from individual components at the site where it is to be produced or applied, the individual components would then need to be measured, combined, and blended using particular methods in order to form proper dispersions. It takes too much time and can be too complex to manufacture a coating composition from its individual components on site. A need, therefore, exists for a less expensive and easier way to provide an aqueous coating to a customer.